Kinetics of oxidative degradation of lignin-based phenolic compounds

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Kinetics, Catalysis, and Reaction Engineering

Kinetics of oxidative degradation of ligninbased phenolic compounds in batch reactor Filipa Manuela Casimiro, Carina A. Esteves Costa, Cidália Maria Botelho, Maria Filomena Barreiro, and Alirio Egidio Rodrigues Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02818 • Publication Date (Web): 08 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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Industrial & Engineering Chemistry Research

Kinetics of oxidative degradation of ligninbased phenolic compounds in batch reactor Filipa M. Casimiro†, Carina A.E. Costa†*, Cidália M. Botelho†, Maria F. Barreiro‡,§, and Alírio E. Rodrigues†

†

Laboratory of Separation and Reaction Engineering - Laboratory of Catalysis and Materials (LSRE-LCM), Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto. Portugal

‡ Laboratory

of Separation and Reaction Engineering – Laboratory of Catalysis

and Materials (LSRE-LCM), Polytechnic Institute of Bragança, Campus Santa Apolónia, 1134, 5301-857 Bragança, Portugal.

§ Centro

de Investigação de Montanha (CIMO), Instituto Politécnico de

Bragança, Campus de Santa Apolónia, 5300-253 Bragança, Portugal

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*Corresponding author: Carina Costa; Laboratory of Separation and Reaction Engineering - Laboratory of Catalysis and Materials (LSRE-LCM), Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal; e-mail: [email protected]

ABSTRACT

Vanillin, vanillic acid, acetovanillone, syringaldehyde, syringic acid, and acetosyringone are products obtained from lignin oxidation in alkaline medium. The evaluation of their individual degradation under oxidation conditions mimicking lignin oxidation is an important tool to better understand this reaction, and maximize the yield of target value-added products. In this context, the main objective of the present work was to study the kinetics of degradation of the selected lignin-based phenolic compounds. The effect of temperature, initial concentration and oxygen partial pressure was evaluated, and a simple 2 ACS Paragon Plus Environment

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mathematical model was developed to describe the data from the degradation of the phenolics during oxidation reactions. The results indicate that, for all the evaluated compounds, the reaction order is first order with respect to both the initial phenolic compound concentration and oxygen concentration. A high degradation rate was found for the reactions performed at 413 K, and an activation energy in the range of 53-86 kJ/mol was found for all the studied phenolic compounds. Moreover, syringic acid is the phenolic compound more prone to degradation, while vanillin is the less one.

1. INTRODUCTION

Lignin is one of the most abundant renewable organic materials on earth, found in wood, annual plants and other vascular tissues.1 It is a heterogeneous macromolecule mainly constituted by three basic units: p-cumaryl, coniferyl, and sinapyl alcohol. These monomers differ in the degree of substitution at the phenolic ring and are the precursors of the main moieties present in lignin structure: H (p-hydroxyphenyl), G (guaiacyl), and S (syringyl) units.2 The ratio



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between G, S and H units in lignin differs among the main groups of plants, namely hardwood, softwood and annual plants.3 Within lignocellulosic biorefineries, lignin conversion to value-added products, such as low molecular weight phenolic compounds, is one of the most straightforward and promising routes to expand lignin applications. In this context, oxidation has been the most studied method for lignin depolymerization.4-9 The oxidative depolymerization of lignin involves the cleavage of aromatic rings, aryl ether linkages and/or other linkages in the lignin structure. Lignin oxidation products range from aromatic aldehydes to carboxylic acids, depending on the lignin source and the harshness of the applied reaction conditions.5, 7, 10 Among the possible routes, oxidation in alkaline medium with oxygen was the main method studied by our research group for lignin valorization.6, 7, 11, 12 Vanillin (V), vanillic acid (VA), acetovanillone (VO), syringaldehyde (Sy), syringic acid (SA) and acetosyringone (SO) are reaction products derived from lignin oxidation. V is widely used as flavoring and fragrance ingredient in food, cosmetic and as a precursor for the synthesis of several second-generation fine chemicals and pharmaceuticals.13,

14

VA, an oxidized form of V, is most often 4

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used as preservative in argan oil and as acidity regulator in wine and vinegar. VO, also known as apocynin, is structurally related to vanillin, and has been studied due to their important pharmacological properties.15-17 Sy is an organic compound occurring widely, in trace amounts, in nature. This phenolic is important in pharmaceutical applications due to their antioxidant, antiinflammatory, antimicrobial and antifungal properties.18 SA is extensively used as therapeutic agent, antioxidant, antimicrobial, anti-inflammatory and anticancer,19, 20

while SO is a chemical compound related to acetophenone and is used as a

plant hormone and insect attractor.21 Nowadays, a great interest is focused on the achievement of ecofriendly processes to obtain V and Sy, as well as other related phenolic compounds (VA, SA, VO, and SO). V production from the oxidation of lignin obtained from side streams of pulp and paper industries and biorefineries is widely studied and reported in literature.6, 7, 11, 22-26 Since the mid 1930’s Tomlinson and Hibbert

23

have reported the production of V from waste

sulfite liquor using a hydrolysis procedure. In the same year, Sandborn and coworkers patented a process to produce V also by a hydrolysis process, but pointed out the need to use oxidation in order to achieve higher production 5 ACS Paragon Plus Environment


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yields.24 Mathias and Rodrigues reported the production of V by oxidation of kraft lignin with oxygen, as well as the effect of the operating process parameters (total pressure, temperature, oxygen partial pressure, lignin concentration and sodium hydroxide concentration) on the kinetics of lignin oxidation reaction.27 In general, the improvement of the reaction yields of phenolics from lignin oxidation, still needs deeper knowledge about the behavior of each individual compound. In literature, there is a lack of studies that report kinetic of oxidation of lignin derived compounds in alkaline medium with oxygen and without heterogeneous catalyst. The main works published use oxidants like H2O2 or ozone and heterogeneous catalyst.14,

28-35

Sales and co-workers studied V

degradation with similar experimental conditions to those used in this work but with a heterogeneous catalyst (palladium catalyst supported on γ-alumina).36 On the other hand, kinetics of V degradation using oxygen in alkaline medium (NaOH) without heterogeneous catalyst was investigated by Fargues and coworkers who developed a kinetic model to evaluate the degradation of this phenolic compound. In the present work an evaluation of the degradation of the main phenolic products derived from lignin oxidation (V, VA, VO, Sy, SA, and SO) 6 ACS Paragon Plus Environment

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was performed. The kinetic study considering these products individually is of great interest since during oxidation their formation can be simultaneously accompanied by their degradation, process that is dependent on the applied oxidation conditions. Oxidation reactions were made in alkaline medium ([NaOH] = 80 g/L; pH ≈ 14) with molecular oxygen, and the effect of temperature, oxygen partial pressure, and initial concentration of the phenolics was evaluated. A simple mathematical model was developed to describe phenolic compounds degradation using general PROcess Modelling System software (gPROMS). The study was completed through the determination of activation energy (Ea) and reaction rate constants (k) for all the studied low molecular weight phenolic compounds.

2. MATERIAL AND METHODS

2.1 CHEMICALS



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Phenolic compounds standards (V, VA, VO, Sy, SA, and SO) were supplied by Sigma – Aldrich (Madrid, Spain), all of them with a purity grade higher than 95 %. Methanol (grade ≥99.8%), formic acid (grade ≥ 99.9%), and sodium hydroxide were provided from Merck (Darmstadt, Germany), oxygen (Alphagaz1 O2 L50, 10.6 m3) and nitrogen (Alphagaz1 N2 L50 9.4 m3) were supplied from Air Liquid (Maia, Portugal).

2.2 PHENOLIC COMPOUNDS OXIDATION IN BATCH REACTOR

Oxidation experiments were performed in a Büchi AG laboratory autoclave with a capacity of 1 L (model BEP280 type II, Switzerland). The heating and temperature control was assured by a Haake thermostatic bath (model N2-B, Karlsruhe, Germany). Temperature and total pressure inside the reactor were measured using a thermocouple type K and a pressure transducer (Kulite model XYME-190 M G, Leonia, USA), respectively. The O2 flow rate was measured by means a mass flow meter EL FLOW (Bronkhorst High-Tech B.V., model F-201CFAC-11-V, Ruurlo, Netherlands) and expressed as standard cubic centimeters


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per minute (sccpm). The reaction samples were collected at preset time intervals through an electro valve (Asco Netherlands) assisted by a universal fraction collector Eldex (model 1243, Napa, USA). The signals of the thermocouple, flowmeter, and pressure transducer were recorded using an acquisition board and LabView program. Experimental set-up is presented in Figure 1. For the oxidation reactions, a selected amount of each phenolic compound (V, VA, VO, Sy, SA or SO) was dissolved in 500 mL of NaOH (80 g/L) solution, introduced into the reactor, heated to the defined temperature, and pressurized. The reaction starts with the admission of O2. Samples from the reaction mixture were collected at regular time intervals. The analyses of the products were performed by the procedure and equipment/conditions as described in the following section.



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Figure 1 - Experimental set-up for kinetic studies of lignin-based phenolic compounds oxidation

2.3 HPLC ANALYSIS

Quantification of V, VA, VO, Sy, SA and SO was made by high performance liquid chromatography (HPLC), as described elsewhere.37 A Knauer HPLC system equipped with a Smartline 5000 online degasser, a Smartline 1000 quaternary pump, and a Smartline 2600 UV-DAD detector was used. The analytical column was an ACE 5 C18-pentafluorophenyl group (250 x 3.0 mm, 5 10 ACS Paragon Plus Environment

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µm) with a guard column of the same material. The detection wavelength was set to 280 nm and the injection volume was 20 µL. Chromatograms were acquired at 30 ºC with a flowrate of 0.6 mL min-1 using an elution gradient composed by two eluents: A) methanol:water (5:95 % v/v) and B) methanol:water (95:5 % v/v), both acidified with formic acid (0.1 % v/v). The elution gradient was [0-3.30] min 90% A; 6.70 min 80% A; [6.70-20] min 80% A; 35 min 50% A; 38.3 min 0% A, [38.341.7] min 0% A; 45 min 90% A; [45-55] min 90% A. Quantification of all compounds was done based on calibration curves previously prepared from standard solutions in the needed concentration range.

3. RESULTS AND DISCUSSION

3.1 KINETIC ANALYSIS OF V, VA, VO, SY, SA, AND SO DEGRADATION

The understanding of the reaction pathways for lignin oxidation can be improved through a preliminary study of the oxidation of model compounds.32 Mathias

11

developed a simple model for V oxidation in alkaline medium with



oxygen, while Fargues, et al.

38

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reported a kinetic study of V oxidation using the

same conditions. In this work, the proposed mechanism for V, VA, VO, Sy, SA, and SO oxidation in alkaline medium with oxygen is based on previously published works.11, 12, 32, 38

Considering the proposed mechanism, phenolics are dissolved in an alkaline

medium to guarantee a high pH (≈14), which implies the dissociation of the phenolics into the ionic forms, and thus react with molecular oxygen. The oxidation of phenolics results from the action of HOO∙ or O2∙ - radicals on the radical A∙, or from the direct action of molecular oxygen. The degradation mechanism of the phenolic compound A, which can be V, VA, VO, Sy, SA or SO, can be represented as:

Initiation

AH + O2 → A∙+ HOO∙

Propagation

A∙ + O2 → AOO∙ AOO∙ + AH → AOOH + A∙

Termination

AOO∙ + AOO∙ → Products


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The equation used to relate the reaction rate with the phenolic compound concentration and oxygen concentration in the liquid phase, [Oliq ], is presented 2 below,

n

rA = k [A]

[Oliq ]m 2

(1)

where, rA is the reaction rate of the phenolic compound, [A] represent the phenolic concentration (V, VA, VO, Sy, SA or SO), k is the oxidation rate constant, and n and

], respectively. The values m the reaction orders with respect to initial [A] and [Oliq 2 of pO2 of the gas phase were converted to concentration of O2 in the liquid phase using a correlation published by Mathias,11 equation (2).

[Oliq ] 2

(

= 3.559 - 6.659 * 10 2

3

* T + 1.498 * 10 *

-3

* T - 5.606 * pO2 + 1.594 * 10

pO2 T

-5

* pO2

)

-0.144 * I -3 * (10 ) * (10 ) in mol L

(2)



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where I is the ionic strength of the solvent in mol/L. The kinetic model developed was applied to describe the degradation of phenolic compounds in batch reactor during oxidation. The model was solved using general PROcess Modelling System

(gPROMS)

(PSA

Enterprise,

United

Kingdom)

and

the

Differential‐Algebraic equation SOLVer (DASOLV), was used to solve the ordinary differential equation in time. To developed the model, different assumptions were considered: a) isothermal conditions, b) perfectly mixed reactor, c) constant liquid and gas volumes, d) closed system with respect to the liquid phase, e) constant total pressure, f) mass transfer resistances negligible, and g) degradation reactions irreversible. The material balance, described in equation (3), is defined taking into account the previous assumptions:

𝑑[𝐴] 𝑑𝑡

= ― 𝑟𝐴

(3)

where, t is the reaction time. Since the starting liquid mixture consists in an aqueous alkaline solution of the phenolic compound with known concentration, the initial conditions for the material balance are: t = 0 and [A] = [A]i, where [A]i is the initial concentration of each phenolic. The performed oxidation reactions were


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simulated, fitting the developed kinetic model to the experimental data, and the results are presented in the following sections. Moreover, it can be stated that the developed kinetic model fits all the experimental results concerning the oxidation reactions of the phenolic compounds.

3.2 EFFECT OF THE PHENOLIC INITIAL CONCENTRATION

The effect of the initial concentration on the degradation of the selected phenolic compounds (V, VA, VO, Sy, SO, and SA) was evaluated though different oxidation reactions performed at constant temperature, T = 393 K, constant oxygen partial pressure, pO2 = 3.1 bar, and constant total pressure, PT = 9.8 bar. The initial concentration of each phenolic compound ranged between 1.3 – 5.0 g/L for V, VA and VO and 0.75 – 2.5 g/L for Sy, SA and SO. The alkaline medium contained a [NaOH] = 80 g/L. The solubility of each compound influenced the selected concentration range, reason why the derivatives from S units were tested at lower initial concentrations than the G units derivatives.



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Figure 2 shows the effect of initial concentration on the degradation yield, for each studied phenolic compound. For V, VA and VO it was verified that initial concentration does not affect degradation yields. After 100 minutes of reaction time, V has a degradation level comprised between 14 and 17 %, independently of the used initial concentration. After the same reaction time, VA shows a complete degradation in all the performed oxidation reactions, being the most reactive compound considering G units derivatives. VO shows a degradation level between 23 and 28 % for the performed oxidation reactions, which means that initial concentration does not significantly affect its degradation. In a previous study addressing the kinetics of V oxidation, Fargues and co-workers also stated that the initial concentration does not affect V degradation yield, i.e. the degradation is almost the same independently of the tested initial concentration.38 Concerning Sy, SA and SO, a similar behavior was observed; the initial concentration of these phenolic compounds does not impact their degradation yield. Sy shows a degradation around 84 % after 100 minutes of reaction time independently of the used initial concentration. Considering SA, it appears to be a very reactive compound, with a complete degradation in less than 15 minutes 16 ACS Paragon Plus Environment

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for all the experimental oxidation conditions. SO also showed a complete degradation, whatever the used initial concentration, but only after surpassing a reaction time of 100 minutes.



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Figure 2 - Effect of initial concentration on degradation along oxidation reaction time: (a) V, (b) VA, (c) VO, (d) Sy, (e) SA, (f) SO and the line represents the fitted kinetic model ( ― ). Initial reaction conditions: [NaOH] = 80 g/L; pH≈14; Ti=393 K; pO2=3 bar; PT=9.8 bar.

3.3 EFFECT OF OXYGEN PARTIAL PRESSURE

To evaluate the effect of the oxygen partial pressure, pO2 on the phenolic compounds (V, VA, VO, Sy, SA, SO) degradation, during oxidation in batch reactor, oxidation reactions were performed in alkaline medium, [NaOH] = 80 g/L, at constant total pressure, PT = 9.8 bar, constant temperature, T = 413 K, and constant initial concentration of the phenolic compound, 2.5 g/L. The tested pO2 was 2.0, 3.1 and 5.1 bar. The concentration of V, VA, VO, Sy, SA and SO versus reaction time, for each oxidation performed with different pO2 levels is shown in Figure 3. The results indicated that VA, Sy, SA and SO have a complete degradation after 80 minutes of reaction, independently of the used pO2. In the case of V and VO the degradation was not complete even after 140 minutes of reaction, independently 18 ACS Paragon Plus Environment

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of the used pO2. Moreover, in the reaction conducted under a pO2 of 5 bar, 44 % of the initially used V was degraded after 80 minutes, while for the reactions using a pO2 of 3 and 2 bar, for the same reaction time, 32 % and 26 % of degradation was obtained, respectively. Considering VO, it was also observed that the increase in pO2 enhances the degradation of this phenolic; in the reaction with 2 bar of pO2, only 42 % of the initial VO was degraded after 80 minutes, comparatively with 55 % and 70 % when 3 and 5 bar of pO2 was used, respectively. In general, the increase of pO2 led to an increase in the degradation of the phenolic compound.



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Figure 3 - Effect of oxygen partial pressure on degradation as a function of reaction time: (a) V, (b) VA, (c) VO, (d) Sy, (e) SA, (f) SO and the line represents the fitted kinetic model ( ― ). Initial reaction conditions: [NaOH] = 80 g/L; pH ≈ 14; initial concentration of compounds 2.5 g/L; Ti = 413 K; PT = 9.8 bar. 20 ACS Paragon Plus Environment

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3.4 EFFECT OF TEMPERATURE ON THE REACTION RATE

In this series of experiments V, VA, VO, Sy, SA and SO oxidation was performed in alkaline medium, [NaOH] = 80 g/L], at constant total pressure, PT = 9.8 bar, constant oxygen partial pressure, pO2 = 3.1 bar (except for Sy, SA and SO, pO2 = 3.0 bar) and an initial concentration of 2.5 g/L. Temperature was 373, 393 and 413 K. The results showed that the increase of temperature enhanced the degradation of all studied phenolic compounds (Figure 4). When an initial T of 413 K was used, a degradation of 24%, 100% and 39% was found for V, VA and VO, respectively, after 1 hour of reaction. It can also be observed that VA is the compound most susceptible to degradation, with 41 % of degradation before 60 minutes of reaction when an initial temperature of 373 K was used. With a temperature of 413 K it was observed a complete degradation for SA and SO at the end of a reaction time of 1 hour, and a degradation of 94 % for Sy. Lower degradations were detected when a temperature of 373 K was used,



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achieving a degradation of 46 %, 100 % and 57 % for Sy, SA and SO respectively after one hour of reaction time. In the oxidation of phenolic ketones (VO and SO) a small amount of the corresponding phenolic aldehydes, V and Sy, respectively, were produced. This is most noticeable when the reactions are performed using an initial temperature of 413 K. Fung and Hrutfiord

39

published a patent concerning the alkaline

oxidation of VO to produce V, where it was stated that the applied reaction conditions affected the conversion yield. Alunga and co-workers have also reported V and Sy formation through VO and SO oxidation, respectively.32 The authors used CuSO4 as the catalyst on the oxidation reactions concluding that the rate of V formation increases with the use of higher temperatures.


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Figure 4 - Effect of temperature on degradation as a function of oxidation reaction time: (a) V, (b) VA, (c) VO, (d) Sy, (e) SA, (f) SO and the line represents the fitted kinetic model ( ― ). Initial reaction conditions: [NaOH] = 80 g/L; pH ≈ 14; initial concentration of compound 2.5 g/L; pO2 = 3.0 bar; Ptotal = 9.8 bar. 23 ACS Paragon Plus Environment


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3.5 KINETIC PARAMETERS

The kinetic parameters used to develop the model that fit the experimental data were described in detail in section 3.1. The reaction orders, n and m, from equation (1), were determined from de slope of the logarithmic representation of

r0 (initial reaction rate for different T) as a function of the logarithmic phenolic initial concentration and logarithmic [𝑂𝑙𝑖𝑞 2 ], respectively (Support Information). The results showed that, for all the studied phenolics, V, VA, VO, Sy, SA and SO, the reaction order with respect to the phenolic initial concentration, was 1. The first order dependence was also obtained for [Oliq . Fargues and co-workers have also 2 ] found a reaction order of 1 with respect to V concentration and to [𝑂𝑙𝑖𝑞 2 ], whereas Alunga and co-workers reported that the kinetic reaction for VO degradation was also of first order.32, 38 The equation describing the reaction kinetics for any of the compound A studied, V, VA, VO, Sy, SA and SO, is given by:

1

𝑟𝐴 = 𝑘[𝐴]1[𝑂𝑙𝑖𝑞 2 ] in g/(L∙min)

(4) 24


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As stated before, temperature has a significant impact on phenolic compounds degradation and its dependence can be explained by the Arrhenius equation (4), which was applied for the determination of kinetic parameters.

Ea

𝑘 = k0 e - RT

(5)

where T is the absolute temperature (in kelvins); k0 is the pre-exponential factor;

Ea is the activation energy for the reaction (in J∙mol−1); and R is the universal gas constant. Equation (4) can be linearized as:

ln (k) = ln (k0) -

Ea1 RT

(6)

The Arrhenius plot, ln (k) versus 1/T, for V, VA, VO and Sy, SA, SO degradation kinetics is presented in Figure 5. The slope from the graphic representation of the logarithm of k (T) as a function of 1/T allows determine Ea for each phenolic compound.



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Figure 5 - Arrhenius plot, logarithm of k versus 1/T, for the calculation of the activation energy of (a) V, VA, VO and (b) Sy, SA, SO oxidation.

Table 1 summarizes the obtained activation parameters for V, VA, VO, Sy, SA, and SO oxidation in alkaline medium with oxygen. The calculated Ea values for V (86.0 kJ/mol) is similar to the one determined by Fargues and co-workers or Wallick and Sarkanen.38, 40 Also Alunga and co-workers reported, in a study about VO degradation with heterogeneous catalyst, a value of Ea for VO, 85.3 kJ/mol, similar to that obtained in this work.32 Kalyani and Jamunarani investigated Sy oxidation with hexacyanoferrate(III) in alkaline medium and reported a Ea of 48.40 kJ/mol.41


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Table 1 – Summary of activation parameters for V, VA, VO, Sy, SA and SO oxidation in alkaline medium. V

VA

VO

Sy

SA

SO

Ea, kJ mol-1

86.0±1

78.4±1

71.6±1

52.7±1

52.7±1

66.2±1

Ea/R

-10348

-9427

-8610

-6336

-6335

-7956

k0

2.27E+11

4.75E+11

6.84E+09

1.30E+08

3.25E+09

1.74E+10

Concerning the reaction rates, in a general way, the results shown that the increasing in the initial concentration, pO2 or T enhanced the initial reaction rate for all the studied phenolic compounds. The reaction rate range varies between 9.39E-03 g/L∙min-1 and 1.34E+00 g/L∙min-1. The phenolic presenting the highest degradation reaction rate was SA, whereas the one with the lower reaction rate was V.

4. CONCLUSIONS

The study of the degradation of valuable phenolic compounds obtained from lignin oxidation in alkaline medium with O2 was the main objective of this work. 27 ACS Paragon Plus Environment


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The performed kinetic study allowed evaluating the reaction orders with respect to V, VA, VO, Sy, SA, and SO initial concentration, and oxygen concentration, as well as the effect of temperature on the kinetic rate constants. The results showed that the initial concentration of phenolic compounds as well as the oxygen concentration, had no effect on the degradation reaction rate, since a first order dependence for both oxidation parameters was obtained for all the studied compounds. The developed pseudo-first-order model fits well all the experimental data, considering the oxidative degradation of the studied phenolic compounds. The activation parameters were calculated, achieving an activation energy in the range of 53 - 86 kJ/mol was found for V, VA, VO, Sy, SA, and SO. Comparing all the tested phenolic, it was possible to conclude that Sy, SA, and SO have higher degradation rates, in comparison with V, VA, and VO. This is an expected behavior since derivatives from S units are more reactive than the G ones due to the presence of two methoxyl groups in the aromatic ring in the later compounds.

ASSOCIATED CONTENT


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Supporting Information.

Logarithmic representation of initial reaction rate, r0, versus logarithmic V, VA, VO, Sy, SA, and SO initial concentration (Figure S1 – Figure S6) to obtain reaction orders n and m with respect to phenolic compounds and oxygen concentration, respectively (PDF).

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]. Tel.: +351 22 041 3658.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT



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This work is a result of: Project “AIProcMat@N2020 - Advanced Industrial Processes and Materials for a Sustainable Northern Region of Portugal 2020”, with the reference NORTE-01-0145-FEDER-000006, supported by Norte Portugal Regional Operational Programme (NORTE 2020), under the Portugal 2020 Partnership Agreement, through the European Regional Development Fund (ERDF); Associate Laboratory LSRE-LCM - UID/EQU/50020/2019 - funded by national funds through FCT/MCTES (PIDDAC) and Foundation for Science and Technology (FCT, Portugal). CIMO (UID/AGR/00690/2019) through FEDER under Program PT2020.

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Kinetics of oxidative degradation of lignin-based phenolic compounds

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